In the case of rewritable optical disk, there was provided in general a method in which some groove or land (groove shape) type steps are set on a plastic substrate as a laser beam tracking and information is recorded at the groove or land. However, in order to improve a density in recording (a narrow trackpitch), a method for recording information at each of groove and land under utilization of the groove and land shapes has been developed in recent years. In this case, the protruded shape in the groove and land is called as a land and the notch shape of the groove and land is defined as a groove. In general, in the case that a track pitch in the information recording medium is changed into a narrow track pitch down to about 80% of a laser beam spot and information is recorded at both land and groove, a leakage of reproducing signal is produced from the adjacent track (either the groove against the land or the land against the groove). For example, when information recorded at the land is reproduced, the reproducing signal from information recorded at the groove is leaked to generate a problem that information recorded at the land cannot be accurately reproduced. The leakage of the reproduced signal from information recorded in the adjacent track is called as a cross-talk.
In order to solve this problem, it is well known in the art to provide a method in which a plurality of magnetic layers are arranged to act as a recording layer and information only at the high temperature section is reproduced under utilization of temperature distribution at the recording layer when the laser beam is radiated (the prior art 1: Japanese Patent Laid-Open No.Hei 8-249737, the prior art 2: Japanese Patent Laid-Open No.Hei 9-293286). In accordance with these methods, since a temperature at the recording layer in the adjacent track becomes a low temperature as compared with the temperature at the recording layer of the center track, no leakage of the reproducing signal from information recorded in the adjacent track is generated. However, such a recording system as above does not provide any sufficient measure against a method for restricting a phenomenon (a so-called cross-erase) in which information (a recording mark) recorded in the adjacent track (the adjacent groove at the time of recording at the land or the adjacent land at the time of recording at the groove) is erased. For example, although the prior art 1 has a heat sink layer with a high thermal conductivity at a side opposite to the laser beam incident side, a distance between the magnetic layer and the heat sink layer is short with 20 nm or less, so that heat is easily dispersed at the adjacent track through the heat sink layer. In addition, although the distance between the magnetic layer and the heat sink layer in the prior art 2 is relatively thick of 80 nm, there are provided three metal magnetic layers as the recoding layer (a functional thin film changing in atomic arrangement or changing in electronic state under application of radiation of the laser beam and a total film thickness of the three layers has a quite thick thickness of 145 nm. Due to this fact, it has been found that heat is dispersed within the recording layer to delete information recorded in the adjacent track.
Accordingly, it has become a substantial problem that a technology for reducing a cross-erase is developed.
However, a step (a groove depth) between a center of the groove and a center of the land in the groove and land shape is usually set to λ/8, where λ is a wavelength of laser forming a laser beam. A reason why this value is set consists in the fact that the largest tracking error signal can be attained when the groove depth is λ/8. However, there sometimes occurs a case that the groove depth is λ/7 or more and λ/5 or less due to the fact that a land groove recording system has been developed recently (prior art 3: Japanese Patent Laid-Open No. Hei 6-338064). A feature of this system consists in the fact that a cross-talk from an adjacent track (a leakage of signal from an adjacent track) can be cancelled even if a track pitch is made to be narrow by about 60% of that of the laser beam spot. However, this recording system does not provide a sufficient measure about a method for restricting a phenomenon (a so-called cross-erase) for deleting a recording mark recorded in the adjacent track (the adjacent groove at the time of land recording or the adjacent land at the time of groove recording). For example, in the prior art 3, although GeSbTe phase change recording material with a low thermal conductivity as compared with metal is used as a recording layer and further the film thickness of the recording layer is 5 to 50 nm, so that it has been found to generate a problem that a distance between the recording layer and the heat sink layer (a reflective layer) has a low value of 18 nm to cause heat to be dispersed into the adjacent track through the heat sink layer at the time of recording of information and a cross-erase is easily generated.
For example, under the standard of DVD-RAM with 2.6 GB/screen, since laser with a laser wavelength (λ) being 645 to 660 nm is metered with a lens having a number of aperture (NA) of lens of 0.6, it is possible to perform a recording and a reproduction under application of the laser beam spot of 0.97 to 0.99 μm (0.9λ/NA). Due to this fact, it is also possible to cancel the cross-talk even in the case that the track pitch is set to 0.7 μm or less. To the contrary, the track pitch under the standard of DVD-RAM with 2.6 GB/screen is set to 0.74 μm. Its reason consists in the fact that a cross-erase is generated in the case that the track pitch is made narrower than this value.
Accordingly, development of a technology for reducing the cross-erase is a substantial problem.
Additionally, a film structure of rewritable type optical disk is a multi-interference structure in which a dielectric member protective layer (hereinafter called as an under protective layer) such as Si2N4, ZnS—SiO2, a recording layer represented by TbFeCo type magnetic film or a chalcogenide type phase change film such as GeSbTe and the like, a dielectric member protective layer similar to the under protective layer (hereinafter called as a heat sink control layer) are laminated in sequence on a transparent substrate made of the plastic material and further there is provided a metal reflection film (hereinafter called as a heat sink layer) made of Al alloy, Au alloy and the like. A feature of this structure consists in attaining a high carrier wave to noise ratio by insisting a variation in optical characteristic value in the recording layer and reflecting light through setting refraction rates of each of the under protective layer, the recording layer, the heat sink control layer and the heat sink layer and setting of the film thickness to a proper value.
A function of the heat sink layer consists in reflecting light passed through the under protective layer, the recording layer and the heat sink control layer and in returning it to the incident side. Accordingly, it is required that its reflectivity is optically high. However, in general, a thermal conductivity of high reflectivity metal such as Al, Au, Pd, Pt, Cu and Ag or the like is quite high, so that some problems are generated as follows.
In the case that the heat sink layer has a high thermal conductivity, it means that heat generated at the recording layer may easily be dispersed into the heat sink layer, so that temperature at the recording layer is hardly increased and a laser power required for recording operation is increased (the recording sensitivity is reduced) (a problem of recording sensitivity).
In order to solve the problems, it is well known in the art to provide a method in which the two heat sink layers are arranged, a thermal conductivity of the heat sink layer near the recording layer is set as a low thermal conductivity and the side of the heat sink layer is provided with a heat sink layer having a high relative thermal conductivity (the prior art 4: Japanese Patent Laid-Open No.Hei 3-272032).
However, since the reflectivity of the low thermal conductivity metal satisfying the thermal conditions was 60%, the low thermal conductivity metal film could not sufficiently satisfy the optical characteristic as the reflective film, resulting in that a signal encoding degree of the reproducing signal, CNR ratio (a carrier wave to noise ratio) and an entire reflectivity of the multiplex interference structure or the like were hardly set to sufficient higher values (a problem of low CNR).
In addition, in the case that the heat sink layer has a high thermal conductivity, there may occur a phenomenon (a cross-erase) that heat is easily dispersed into a direction of plane of the heat sink layer and under influence of this heat, information recorded at the adjacent track is deleted (a problem of the cross-erase).
In addition, when the recording film of the rewritable type optical disk is made of phase change recording material, these information recording media are comprised of a protective layer, a recording film such as GeSbTe, a protective layer and a reflection area of GeSbTe on the substrate, and the reflectivity at the crystalline state is higher than that of amorphous state. With such an arrangement as above, the absorbing rate at the recording film shows a higher value at its amorphous state. If an over-writing is carried out under this state, the recording mark under its amorphous state may absorb light more easily than its crystalline state, resulting in that its temperature may easily be increased, and a size of the mark newly recorded is increased more than its normal size and a certain strain is generated at the reproducing signal.
In order to prevent this phenomenon, a trial has been accomplished to increase an absorbing rate under a crystalline state at the recording film than that of its amorphous state. For example, there are present a case in which a relation between an absorbing rate under a crystalline state and an absorbing rate under an amorphous state is reversed under an arrangement of a quite thin Au reflective layer with a thickness of 10 nm (the prior art 5: Shingaku Technical Bulletin MR92-71, CPM92-148 (1992-12) P.37) or a case in which a relation between an absorbing rate under a crystalline state and an absorbing rate under an amorphous state is reversed under an arrangement of Si at a reflective layer with a thickness of 65 nm (the prior art 6: Shingaku Technical Bulletin MR93-53, CPM93-105 (1993-12) P.1).
However, since any of these methods has no heat sink layer of sufficient thickness, it is not possible to perform a fast cooling of the recording film after the recording film is melted. Due to this fact, there occurs a problem that the recording film is deteriorated after performing the re-writing operation for several times (a deterioration problem of recording film when the re-writing operation is performed by several times).
In addition, in order to solve the problems above, there is provided a method in which two reflective layers are arranged at a side of the recording film opposite to the laser beam incident side, the reflective layer near the recording film is applied as Si and the other reflection film (a heat sink layer) is applied as an Al alloy, thereby a relation between the absorbing rate under the crystalline state and the absorbing rate under the amorphous state is reversed (the prior art 7: Proceeding of International Symposium on Optical Memory 1995, pp 151-152). Although this method is a superior one, reversing of the absorbing rate cannot be carried out sufficiently; since a thickness of the Si thin film must be limited to 50 nm to 100 nm in order to attain an optical superior multi interference structure, so that a degree of freedom in heat design is reduced; in addition, since there is provided a sufficient heat sink layer (a protective layer present between the recording layer and Si thin film), heat is dispersed to the adjacent track through the Si thin film having a high thermal conductivity and a heat sink layer (Al alloy) to generate the cross-erase. Further, since the semiconductor film such as Si or the like has normally a low film-forming rate, its productivity is not superior and it shows a certain problem in its production.
As described above in detail, when a high-density recording is carried out, various kinds of problems may occur. In particular, in the case that a high-density recording is carried out with a track pitch being less than 70% or less, a technology for remarkably reducing the cross-erase is an essential one.
Although all the methods were superior methods, a high-density recording to have the track pitch of 70% or less of the laser spot diameter was not sufficiently assumed such that the track pitch becomes 70% or less of the laser beam spot diameter, resulting in that all these methods could not reduce the cross-erase sufficiently. In addition, it is apparent that the structure where the cross-talk may easily occur is a structure in which a flow of heat from the land to the groove or from the groove to the land is substantially high. Further, in the case that the flow of heat between the land and the groove is high, there occurs a problem that the laser power required for the recording operation is different for the case of recording at the land and for the case of recording at the groove, respectively, when information is recorded at both land and groove as described later.
It is an object of the present invention to provide an information recording medium in which CNR, an over-writing characteristic and a recording sensitivity are not reduced even under a high-density recording in which the track pitch becomes 70% or less of the laser beam spot diameter and further no cross-erase is produced.
Further, it is another object of the present invention to enable a narrow track pitch to be realized even in a high-density recording in which the track pitch becomes 70% or less of the laser beam spot diameter without reducing CNR, an over-writing characteristic and a recording sensitivity and to realize an information recording medium of high recording density corresponding to an information recording and reproducing apparatus using a low-cost semiconductor laser.
Further, it is another object of the present invention to enable a narrow track pitch to be realized even in a high-density recording in which the track pitch becomes 70% or less of the laser beam spot diameter without reducing CNR and a recording sensitivity and in particular to provide an information recording medium in which a superior over-writing characteristic is realized even under a high-density recording where a recording mark length may become a half or less of an energy beam spot diameter.
Further, it is a further object of the present invention to enable a narrow track pitch to be realized even in a high-density recording in which the track pitch becomes 70% or less of the laser beam spot diameter without reducing CNR, an over-writing characteristic and a recording sensitivity and to provide an information recording medium in which a reproducing signal is not deteriorated even after many times of re-writing operation of about 100,000 times.
Further, it is a still further object of the present invention to enable a narrow track pitch to be realized even in a high-density recording in which the track pitch becomes 70% or less of the laser beam spot diameter without reducing CNR, an over-writing characteristic and a recording sensitivity and further to provide an information recording medium in which no difference is produced in a recording sensitivity between a case having information recorded at the groove and a case having information recorded at the groove.